It is known in the art to realize dipole antennas of the substantially-linear half-wave linear type, that is to say composed of two wire-shaped aligned branches, preferably axially disposed, in electrically conductive material whose whole length is equal to ½ of the wavelength to be received or transmitted, adapted to be housed inside a safety helmet to thus allow the radio signal reception or transmission by a radio equipment, the latter being arranged too inside the safety helmet.
As it is well known, the use of a substantially-linear dipole antenna for the transmission and reception of radio signals in a safety helmet is particularly popular for the optimal omnidirectionality features shown by such type of antennas, and for their constructive easiness, and finally for the sizes of such antennas which, in the bandwidth (2.4-2.5 GHz) commonly used in the vehicular radio transmissions, are particularly reduced and therefore easily adaptable to the shape of the helmet outer cap.
However, just their reduced sizes, on the order of 3 cm for each branch composing the dipole antenna in the case of 2.4-2.5 GHz frequencies, and their arrangement inside the cap in a central region thereof, so that asymmetries in the reception/transmission of radio signals do not occur, cause such antennas to exhibit a reduced reception/transmission area (range), due to the interference of the user head and neck, when the helmet is correctly worn.
It is in fact well known that at typical operative frequencies of the vehicular transmissions, such as for example those of “Bluetooth” radio standard equal to about 2.45 GHz, the maximum signal absorption at such frequency band is given by water and therefore by the human body.
Note as well that the position of such type of substantially-linear half-wave dipole antenna in a safety helmet, for example for motorcyclists, is usually limited to a central, back and bottom region of the helmet outer cap, between such outer cap and the shell thereof made in shock-absorber material, both for constructive convenience reasons, and for bulk reasons.
In such a specific position, the signal absorption at the frequency band comprised between 2.4 and 2.5 GHz by the human body, and in particular by the user head and neck, is particularly significant and can reduce the range of the antenna from half of its theoretical range to one third of such a range.